In fluorescence observation devices, such as fluorescence endoscopes, there are known conventional techniques in which a number of pixels for binning summing or an exposure time is changed according to the brightness of an image in order to detect weak fluorescence from a subject, and, if the amount of fluorescence is low, the number of pixels for binning summing is increased, or the exposure time is lengthened, thereby increasing the sensitivity for observation (for example, see PTL 1).